This invention relates to a cover for a push button switch used as a data input unit or a switch unit for a mobile communication equipment such as a mobile phone, a vehicle-mounted telephone or the like, a measuring instrument, a remote controller, a handy terminal or the like, an input unit in the field of a domestic electric appliance, an electronic equipment, a communication equipment, or the like. More particularly, the present invention relates to a push button switch cover which permits the push button switch to exhibit improved keying characteristics such as reduced stroke characteristics, increased click characteristics and the like.
Now, a conventional push button switch which is used for an input unit for a mobile phone or the like will be described with reference to FIG. 14. The push button switch includes a cover 100, which is received in a casing of the equipment or phone while being mounted on a circuit board 102 through dish-shaped springs 104 made of metal such as phosphor bronze, SUS stainless steel or the like, resulting in the push button switch being constituted. The circuit board 102 is provided thereon with contacts 106. In the push button switch thus constructed, displacement of each of key buttons 108 toward the circuit board 102 permits a corresponding one of pressing projections 110 to displace the dish-shaped metal spring 104.
Several or tens of such dish-shaped metal springs 104 are arranged in perforated portions of a perforated PET sheet 112 having a pressure-sensitive adhesive or an adhesive coated thereon by means of an aligning unit such as a parts feeder or a robot, a jig, or the like while being kept projected. Then, a pressure-sensitive adhesive sheet 114 is positioned over the dish-shaped metal springs 104 to fix the dish-shaped metal springs 104 between the perforated PET sheet 112 and the pressure-sensitive adhesive sheet 114, resulting in a dish-shaped metal spring sheet being provided, which is then superposed on the push button switch cover 100.
However, the conventional push button switch cover 100 thus constructed is provided separately from the dish-shaped metal springs 104. This causes alignment between each of the pressing projections 110 of the cover 100 and an apex of a corresponding one of the dish-shaped metal springs 104 during superposition of the dish-shaped metal spring sheet on the cover 100 to be highly difficult or troublesome. Also, a procedure for inspecting such alignment has not been established in the art. Thus, the conventional push button switch cover fails to provide the push button switch with satisfactory keying characteristics.
In particular, positioning of the perforated PET sheet 112 mounted thereon with the dish-shaped metal springs 104 with respect to the push button switch cover 100 for superposition of the former on the latter is carried out by inserting positioning pins of an aligning jig through reference holes of the perforated PET sheet 112 and cover 100. Unfortunately, this causes positional deviation, at the most, between 0.2 mm and 0.5 mm to occur between the PET sheet 112 and the cover 100 due to misregistration between the reference holes and gaps between the reference holes and the positioning pins. Also, the prior art fails to permit accuracy of positioning between the apex of each of the dish-shaped metal springs 104 and each of the projections 110 of the push button switch cover 100 to be confirmed. Thus, the push button switch cover is assembled while keeping positional accuracy between the apex of the dish-shaped metal spring 104 and the projection 110 of the cover 100 from being increased, resulting in the push button switch cover being deteriorated in keying characteristics such as keying load, pressing feeling and the like.
In addition, the prior art requires the PET sheet 112 for holding the dish-shaped metal springs 104 and the pressure-sensitive adhesive sheet 114 fixed on the circuit board 102 are also required, leading to an increase in manufacturing cost.
Moreover, in the conventional push button switch cover, the pressing projections 110 are each compressedly deformed by impact resilience of the dish-shaped metal spring 104, to thereby cause a stroke generating peak load f1 (peak stroke s1) and a make stroke s2 generating make load f2 to be increased as indicated at a thin solid line B in FIG. 8, resulting in operation feeling being excessively soft and lacking clearness or distinctness. The ratio of differential load between peak load and make load to peak load will be referred to as xe2x80x9cclick ratioxe2x80x9d hereinafter.
In order to provide operation feeling with increased clearness, it is desired to eliminate compressive deformation of the pressing projections 110 to coincide press load (gf)-stroke (mm) characteristics of the whole cover with ideal characteristics of the dish-shaped metal spring per se as indicated at a broken line C in FIG. 8. It would be considered that the cover is increased in hardness for elimination of compressive deformation of the pressing projections. However, when the dish-shaped metal spring 104 is deformed through a thin wall portion of the cover 100 of, for example, 0.1 to 0.3 mm, in thickness to generate click feeling in the cover shown in FIG. 1, an increase in hardness of the cover 100 tends to cause the cover to be broken due to repeated displacement, leading to a deterioration in keying durability of the cover 100.
In addition, the inventors found that, when the dish-shaped metal springs are bonded to the pressing projections by an elastic adhesive, the push button switch cover causes a stroke with respect to load during keying operation to be increased if the amount of the elastic adhesive is insufficient and causes the stroke to be reduced if the amount is excessive, to thereby reduce a click ratio, leading to a deterioration in bonding durability and functional durability. Also, inclination of the dish-shaped metal spring arranged by means of the adhesive is caused to lead to positional deviation of the dish-shaped metal spring in X-Y directions. For example, peak load is increased above positional deviation of 0.1 mm, resulting in the bonding durability being substantially reduced in association with a reduction in click ratio. A minimum value of inclination of the dish-shaped metal spring which can be visually confirmed is 2.5 degrees.
The present invention has been made in view of the foregoing disadvantages of the prior art.
Accordingly, it is an object of the present invention to provide a push button switch cover which is capable of increasing positioning accuracy with which a cover substrate and a dishshaped metal spring are positioned with respect to each other.
It is another object of the present invention to provide a push button switch cover which is capable of exhibiting satisfactory keying load characteristics, to thereby exhibit increased pressing feeling over a long period of time.
It is a further object of the present invention to provide a push button switch cover which is capable of eliminating arrangement of members such as a pressure-sensitive adhesive sheet for fixing dish-shaped metal springs, a perforated PET sheet and the like, to thereby significantly reduce a manufacturing cost.
It is still another object of the present invention to provide a push button switch cover which is capable of minimizing affection of compressive deformation of pressing projections, to thereby provide satisfactory click feeling and clear or distinct keying feeling.
It is yet another object of the present invention to provide a method for manufacturing a push button switch cover which is capable of facilitating inspection of positional accuracy between pressing projections and dish-shaped metal springs.
In accordance with one aspect of the present invention to provide a push button switch cover is provided. The push button switch cover includes a cover substrate, which is provided on a front surface thereof with at least one key top and on a rear surface thereof with at least one pressing projection in a manner to correspond to the key top. The push button switch cover also includes at least one dish-shaped metal spring arranged on the pressing projection corresponding thereto. The dish-shaped metal spring is fixed on the pressing projection corresponding thereto through an adhesive section in a manner to be abutted at an apex thereof against a central portion of the pressing projection.
Also, in accordance with this aspect of the present invention, a push button switch cover is provided. The push button switch cover includes at least one key top, at least one pressing projection arranged on a rear side of the key top, and at least one dish-shaped metal spring having a dome-shaped top and arranged on the pressing projection corresponding thereto. The dish-shaped metal spring is bonded at the dome-shaped top thereof to a central portion of the pressing projection by means of an elastic adhesive section.
In a preferred embodiment of the present invention, the adhesive section comprises an elastic adhesive having an elongation of 75 to 700% and preferably 75 to 250% in an amount of 2 to 7 mg.
In a preferred embodiment of the present invention, the elastic adhesive has an initial viscosity of 20 to 150 Pa-s and preferably 30 to 100 Pa-s.
In a preferred embodiment of the present invention, the elastic adhesive is constituted of 100 parts by weight of a main adhesive ingredient and 5 to 50 parts by weight of a silicone adhesive ingredient.
In a preferred embodiment of the present invention, the cover substrate is made of silicone rubber having a Shore A hardness of 40 to 70 and the pressing projection is integrally formed on the cover substrate. The elastic adhesive section has a Shore A hardness of 20 to 90.
In a preferred embodiment of the present invention, the pressing projection is integrally formed on the rear surface of the cover substrate. The key top is formed on the front surface of the cover substrate.
In a preferred embodiment of the present invention, the pressing projection is integrally formed on the rear surface of the cover substrate. The key top is made of a resin material and bonded to the front surface of the cover substrate.
In accordance with another aspect of the present invention, a method for manufacturing a push button switch cover is provided. The method includes the steps of: providing a cover substrate which is formed on a front surface thereof with at least one key top and on a rear surface thereof with at least one pressing projection in a manner to correspond to the key top; applying an adhesive to the pressing projection to form an adhesive section on the pressing projection; carrying the cover substrate to a dish-shaped metal spring feed position while keeping the pressing projection facing up; and pressing a dish-shaped metal spring onto the pressing projection corresponding thereto while aligning a central portion of the pressing projection with an apex of the dish-shaped metal, whereby the dish-shaped metal spring is bonded to the pressing projection through the adhesive section.